Fluid and plume geochemical characteristics of Mid-Atlantic Ridge (MAR) hydrothermal fields have been compared. Similar geological traits of Logatchev and Rainbow fields are location of their discharge zones in ultramafic environments. High H 2 and СH 4 concentrations reflect the influence of serpentinization processes on forming of a fluid composition. The concentration of H2S is noticeably less in comparison with fluids of other Atlantic hydrothermal fields. Regression analysis shows that all the sulfur found out in samples from fluids and plumes of both fields is represented in the form of sulphate-ion. This lead to supposition of negative correlation of H 2 S and СH 4 . Sampling of fluids during dives on DSRV ALVIN and plumes from board of RV ATLANTIS let me to calculate the end member composition. It testifies to sharp distinctions in solution composition of Rainbow and Logatchev fields. In case of the Rainbow highly mineralized fluids are characteristic. On the Irina-2 vent of the Logatchev area solutions with distinctly less macrocomponent (first of all Na and Cl) concentrations were sampled as compared to ocean water. Quite noticeable are differences in concentrations of Fe and Mn. For Rainbow fluids the highest contents of Fe (> 1300 mg/l) and Mn (up to 123 mg/l) observed in MAR hydrothermal fields are characteristic. Concentrations of these elements in Irina-2 vent fluids decrease for the order of magnitude.
